As you are using a dual CPU machine I would also expect an improvement when using multiple bshell. But maybe your system is not ideal:
- do you have sufficient memory available for other processes or has Oracle everything?
- is IO maybe a bottleneck?
- is maybe the code path not optimal and do you have locking issues?
Happy digging,
Dick

1- SGA=1,2 Gb
2- When CPUs are less than 100% I see I/O wait, when CPUs are 100% I/O wait is 0.
3- Don't understand: what is code path? Locking issues?